The manufacture of brake discs from, for example, flake graphite cast iron takes place generally according to the sand casting process. In a well-known manufacturing process the brake discs are cast rising vertically in a system, i.e. in an upright standing position. The system is very fast and therefore efficient. It has, however, been found that major differences in structure and hardness occur in the brake discs during this casting process, which lead to considerable deficiencies in quality on brake discs.
The same applies to a likewise well-known manufacture of brake discs using the horizontal casting method. For this, generally box-less systems are used, with the sand being pressed through a frame and subsequently the frame again being ejected or withdrawn. Prior to casting, so-called loading irons are placed on the mold, which is why gating and casting must be carried out one-sidedly from one side via feeding gates. Depending on the type of brake discs, there is a plurality of gates with the risk of tendency toward shrinkholes or blowholes or a circular runner is used.
One of the problems with this process consists in that the fluid iron runs at points into the disc mold. This gives rise to local heating-up at the points in the area of the inlet and a correspondingly slower and uneven or irregular cooling-down. This means that the brake disc is stressed thermally on one side, causing considerable differences in the hardness of the material due to the differing cooling rates. Also of disadvantage is that in ventilated brake discs, i.e. brake discs provided with cooling channels, irregularities occur in the cooling channels. This results in an unbalance of the disc.
Due to the occurring differences in hardness, the parallelism of the two disc surfaces, i.e. the friction rings, also suffers. In practice, hardness differences of up to 20 Brinell hardness points have been found. During machining of the brake disc, these hardness differences lead to turning errors, particularly during turning. For example, during turning with a toolholder which always works at the same loading pressure, dents occur in the surface at the soft or spongy spots.